In the beginning President Obama approached the issue of gay marriage cautiously.

He had long supported civil unions. He had worked to repeal "Don't Ask Don't Tell" and he'd come out against the Defense of Marriage Act (DOMA) which denies federal benefits to same-sex couples legally married in their states.

But even when he became the first sitting president in May of 2012 to say that he thought gays and lesbians should be able to marry, he moved carefully.

When pressed by ABC's Robin Roberts he suggested that the issue should be left to the states. "I continue to believe," he said, "that this is an issue that is going to be worked out at the local level."

But his endorsement influenced a cascade of politicians to switch positions on the issue. Even though 30 states still had language defining marriage as between a man and a woman in their constitutions, by November three more states would authorize gay marriage.

By the time of his inaugural address the president was speaking in sweeping terms.

He said, "Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law--for if we are truly created equal, then surely the love we commit to one another must be equal as well."

For gay rights activists the president's words were deeply moving. "He made clear that support for the freedom to marry is not just important to gay people and our loved ones, but a central part of the American project, our journey toward liberty and justice for all," says Evan Wolfson of the group Freedom to Marry.

By February, as expected, Solicitor General Donald B. Verrilli Jr. filed a Supreme Court brief in US v. Windsor, the DOMA case. Verrilli reiterated the administration's position since February 2011 that the law is unconstitutional. Verrilli wrote that DOMA "denies to tens of thousands of same-sex couples who are legally married under state law an array of important federal benefits that are available to legally married opposite-sex couples."

But at the time, the filing deadline was approaching in another case before the Supreme Court that asked a much broader constitutional question. Hollingsworth v. Perry is a challenge to California's Proposition 8, passed in 2008, that defines marriage as between a man and a woman.

Buoyed by the language in the inaugural address, some gay rights advocates wondered if the Department of Justice would weigh in, even though the federal government wasn't an official party in the case.

"It would be very important for us, for the United States to make clear that this is a matter that affects the lives of all Americans, it's the Constitution for all America," said Theodore Olson, an attorney representing two gay couples challenging Prop 8.

On February 28, Verrilli filed a brief and opponents of Prop 8 were delighted. Verrilli had not called for a constitutional right to gay marriage, but he wrote, "the president and the attorney general have determined that classifications based on sexual orientation should be subject to heightened scrutiny for equal protection purposes." If the court were to agree with would mean that any law based on the classification of sexual orientation would face much tougher scrutiny from the courts.

Verrilli said, "Proposition 8's denial of marriage to same-sex couples, particularly where California at the same time grants same-sex partners all the substantive rights of marriage, violates equal protection. "

The next day Obama had a news conference explaining the decision to file a brief in the case: "What we've said is that same-sex couples are a group, a class that deserves heightened scrutiny, that the Supreme Court needs to ask the state why it's doing it, and if the state doesn't have a good reason it should be struck down."

At the end of March, the Supreme Court heard arguments in both the Prop 8 and the DOMA cases.

Lawyer Charles J Cooper argued on behalf of supporters of Prop 8 and told the justices that Californians who voted in favor of the ballot initiative opted "in good faith" to preserve the traditional definition of marriage because they believe it continues to meaningfully serve important societal interests. Cooper said that the government has an interest in traditional marriage to encourage "responsible procreation."

Cooper pointed out that California has expansive domestic partnership laws that provide gays and lesbians with "comprehensive civil rights protections."

Justice Anthony Kennedy--whose vote could be pivotal--worried on one hand that sociological information on gay marriage is relatively new. "We have five years of information to weigh against 2,000 years of history or more," he said. But on the other hand he expressed concern for the children. "There are some 40,000 children in California," he said, "that live with same-sex parents, and they want their parents to have full recognition and full status."

After arguments the original sponsors of Prop 8 released a statement. "No amount of new-found political or legal firepower can replace the primary purpose of marriage, which is to increase the likelihood of children being raised by the mothers and fathers who brought them into this world. We are confident that the case we presented today will affirm that same-sex "marriage" is but a societal experiment that needs no special protection by or recognition from the United States Supreme Court," the statement said.

In the DOMA case, former Solicitor General Paul D. Clement--who was appointed by House Republicans to defend the law when the government declined to do so--reminded the court that DOMA had received broad support in both the House and the Senate when it passed in 1996. He said Congress acted in part out of a desire for uniformity so that the federal definition of marriage would remain consistent throughout the states.

"For purposes of federal law," Clement said, "it's much more rational ...for Congress to say we want to treat the same-sex couple in New York the same way as the committed same-sex couple in Oklahoma."

In May, Rhode Island and Delaware became the 10th and 11th states to allow gay marriage in addition to the District of Columbia.

By the end of June, the Supreme Court will issue opinions in both the DOMA and Prop 8 cases. For some gay rights advocates the momentum will be hard to reverse no matter how the Supreme Court rules.

"As long as all who believe in justice stay with us in the work ahead, we will win marriage nationwide either in June with the Supreme Court ruling, or after a few more years of engagement when we go back before the court with even more states, still more public support, and perhaps a couple new justices," says Wolfson.

The court's ruling will come almost at the one year anniversary of the president's initial, personal, endorsement of gay marriage.